Field of the Invention
The invention relates to a vehicle control system for a linkage on the vehicle. More specifically, the invention relates to a method for determining a draft force of a tractor and controlling a linkage on the tractor.
Description of Related Art
A hitch, such as a three-point linkage is a known arrangement used to attach implements to a vehicle, for example an agricultural tractor for towing. The implement may be fully-mounted or semi-mounted whereby a semi-mounted implemented has a wheel engaging with the ground during soil operation while a fully-mounted implement puts all its load on the three-point linkage.
Three point linkages most frequently consist of two lower lifting arms to which an implement is attached. The lower lifting arms can be pivoted by respective hydraulic actuating cylinders to adjust the height position of the implement relative to the tractor. Furthermore, these lower lifting arms may be manually adjusted in length and thus be adapted to the type of implement to be attached. An additional top link connects the implement to the tractor on a level above the lower lifting arms. This top link is used to pivot the implement about a horizontal transverse axis and is adjustable by means of a threaded connection, or a hydraulic cylinder.
Alternative designs of three-point linkages are known, such as the arrangements shown in U.S. Pat. No. 6,321,851, US2003/217852 and U.S. Pat. No. 5,997,024 in which the lower links are replaced by two, or four variable length hydraulic rams. This variable length ram arrangement enables multi axis movement of any implement attached to the linkage.
To control the three-point linkage, modern tractors are mainly equipped with electronic linkage control systems to improve the work quality of the machine and the operator's comfort during operation.
Such electronic linkage control systems operate in three well known modes:
Position control mode: In general, the tractor speed is kept constant by a speed control system and the position of the lower lifting arms is sensed directly, or indirectly so that the working depth of the implement in the soil can be adjusted within limits set by the operator whilst the speed of the tractor is kept constant. In the position control mode, the height of the linkage is determined by a function, H which relies solely on a height position component HP inputted or controlled by the operator via a depth controller.
Draft control mode: The implement is raised and lowered in the soil automatically depending on the draft force applied by the implement to reduce fuel consumption, avoid engine stall or avoid damage of the implement or tractor. Again, vehicle speed is kept constant. If the implement is lowered into the ground an initial draft is applied defining a zero level. The operator can then set a value representing a force increase which means that the operator can decide how fast the implement is lifted when a small force increase or a large force increase occurs. The value of the force entered by the operator does not represent an exact value of the force applied, e.g. 5 kN, but defines the responsiveness of the draft control. The objective of this function is to move the implement while avoiding excessive draft or pull force variations. Therefore, a draft force sensor, typically in the form of a draft force sensing pin which connects the lower lifting arms to the tractor chassis is used to measure the horizontal load applied to the tractor by the implement. In the draft control mode, the height of the linkage is determined by a function, H which relies on a height draft component HD resulting from the draft force.
Intermix control mode: This control arrangement, as its name implies is a mixture of position and draft control modes in which a draft control system can only lift the implement within a limited range of positions. This function is provided to avoid excessive movement of the implement in the soil resulting in poor working quality. Again, vehicle speed is kept constant by a speed control system. In the intermix control mode, the height of the linkage is determined by a function H which relies on both the height draft component HD (from the draft control mode) and the height position component HP (from the position control mode). A mix controller can vary the ratio of HD:HP and thus vary the influence from each of the position and draft control modes.
Only the draft control and intermix mode (both referred to as draft modes) operate under measurement of the draft force. Generally, deactivating the draft modes results in that the system enters the position mode with no draft force influencing the lifting heights. It may, however be difficult to install a draft force sensing pin due to the complex three-dimensional geometry of a linkage.
The applicant's pending application WO2013/053645 describes an electronic linkage control wherein the draft force sensing pin is omitted and the variation of the draft force of the linkage is determined from the variation in the hydraulic drive circuit pressure. As described in WO2013/053645, such systems may also be used for implements which are simply towed by connection to e.g. a ball hitch, rather than being hitch mounted. Different to fully-mounted implements the weight of the implement mainly rests on the ground via the implement wheels and actuators are provided on the implements controlling linkage thereon to raise and lower soil engaging means. The actuators, mainly hydraulic actuating cylinders similar to those used for three point linkages, are thereby supplied and controlled by the hydraulic supply system and valve arrangements of the tractor in known manner.
Furthermore, an implement may be semi-mounted, which means that the implement is mounted to the lower lifting arms of the tractor and further may be equipped with a ground engaging wheel so that the weight of the implement rests on the lower links and the ground via the implement wheels. To raise and lower the soil engaging means of the implement, the linkage of the tractor and the linkage of the implement must be adjusted conjointly.
Accordingly, the term “linkage” may be understood as not limited to a three-point linkage of a tractor, but also includes linkages to move soil engaging means of towed implements.
Normally, in the draft mode, the operator sets a value indicative of an acceptable draft force (depending on the condition of the ground and desired vehicle speed). If the draft force then rises continuously because a plough in the ground has hit a rock, the draft control will move the linkage and therefore the plough upwards so that the draft force is reduced. If the draft force is reduced, the control will then lower the linkage and plough back again into ground. In this way, the plough will automatically pass a rock in the ground avoiding damage to it.
However, the determination of the draft force based on CVT (Continuously Variable Transmission) parameters encounters problems in some vehicle operating conditions.
In the case where an implement is attached to a linkage of a tractor, the operator is ordinarily responsible for deactivating the draft or intermix control modes manually when travelling along a road with an implement held in a lifted position. If this is not done the draft force determined by the CVT delivers a significant change of draft signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration. This can result in the implement being lowered to the ground, or lifted when there is no need.
In the case of acceleration to a new speed regardless of the position of the implement, the change of draft signal may result in unintentional movement of the linkage. The faster the vehicle is going, the greater is the risk of damage through unintentional movement of the linkage.